In general, the life span of an organism is defined by measuring its chronological age. Alternatively, the life span of yeasts can be determined by measuring the number of mitotic divisions completed by a mother cell prior to senescence, defined as the replicative life span (“RLS”). To determine the RLS for a yeast mother cell, each daughter cell needs to be physically removed from a mother cell after each mitotic cycle. Yeast strains that are genetically predisposed to a long life span are identified, for example, by measuring a mean RLS for a statistically reliable number of mother cells for each strain, and by comparing the determined mean RLS value to a mean RLS value of a reference population. Alternatively, life spans can be measured by determining the median RLS value observed for a given strain, or by determining the maximum RLS value observed for a strain.
The labor-intensive nature of the RLS assay and the vast number of individual RLS determinations that need to be performed for a statistically significant set of each variant in a library have precluded attempts to comprehensively approach the identification of aging mutations by implementing a genome-wide characterization. For example, because laboratory yeast strains are highly variable, approximately 40 to 50 cells of each genotype are evaluated in order to determine a statistically reliable mean RLS for a particular strain of interest. When 50 cells of each genetic variant from a hypothetical genomic library containing 5,000 variants are used to determine the mean RLS, then a total number of 250,000 RLS determinations would need to be performed. Determinations of such large numbers of RLSs is impractical, and, therefore, comprehensive analysis of a large collection of strains representing variants of a yeast genome have not been carried out.
Prior to the present invention, RLS determinations have been made utilizing yeast strains of highly disparate backgrounds, including different short-lived strains that have a mean RLS less than the mean RLS of other wildtype yeast strains. Since laboratory yeast strains are highly divergent at the genomic level, many mutations identified from these variant strains may affect RLS in a strain-specific manner. For example, mutations that increase the life span of short-lived strains may result from suppression or reversion events that compensate for strain-specific mutations, and such longevity-promoting mutations may be specific to only that particular strain.
FIG. 1 illustrates aging regulatory pathways in eukaryotes. In yeast, at least two biological pathways that regulate life spans are known, a pathway mediated by the SIR2 gene and a pathway responsive to calorie restriction (“CR”). The Sir2 gene product is a NAD-dependent histone deacetylase that regulates the rate at which extra-chromosomal ribosomal DNA circles (ERCs) are formed in the nucleus of a mother cell. The accumulation of ERCs within a mother cell with each successive mitotic event is one mechanism by which yeast cells age. Deletion of Sir2 decreases life span by approximately 50%, and over-expression of Sir2 increases lifespan by approximately 30-40%. The Sir2 ortholog, Sir-2.1, regulates aging in C. elegans through a pathway dependent on the Daf-16 transcription factor. In mammals, a similar pathway exists, in which the Sir2 ortholog, SirT1, regulates the activity of FOXO3a, a Daf-16 ortholog. Although life-span-regulating mechanisms of most eukaryotic organisms, including yeast, worms, and mammals, are highly responsive to calorie restriction (CR), gene products that regulate the CR pathway are, as yet, poorly characterized.
Analysis of yeast variants that are predisposed to longevity can yield previously uncharacterized genes that confer long life spans. An improved method for efficiently evaluating a large collection of genetic variants with differential life spans in order to identify genes that confer longevity is highly desirable. Gene products that regulate the life spans of eukaryotes can be targeted by pharmaceutical agents in order to decrease the rate of aging. Pharmaceutical agents and methods for screening such pharmaceutical agents that can increase the life expectancy of mammals, including humans, are highly desirable. In addition, by slowing the rate of aging, it may be possible to delay the onset of various diseases/conditions associated with aging, including various types of cancers, diabetes, cataracts, heart diseases, and neurodegenerative diseases, such as Parkinson's disease, Huntington disease, and amyloid diseases.